박막 도핑에 의한 태양광 소자의 광기전력 특성

Abstract

Cadmium telluride (CdTe) thin-film solar cell technology is well known that it can theoretically improve its conversion efficiency and manufacturing costs compared to the conventional silicon solar cell technology, due to its optical band gap energy (about 1.45eV) for solar energy absorption, high light absorption capability and low cost requirements for producing solar cells. Although the prior studies obtained the high light absorption, CdTe thin film solar cell has not been come up to the sufficient efficiency yet. The maximum efficiency of a CdTe thin film solar cell still remains just 16.5% despite its excellent absorption coefficient; i.e., the electrical properties of CdTe thin film, including the resistivity, must be improved to enhance the energy conversion efficiency. The electrical properties of CdTe thin films are essentially determined by the doping process; so, doping method was selected for the improvement of the electrical characteristics in CdTe solar cells. Some elements including Cu, Ag, Cd and Te were generally used for the p-dopant as substitutional acceptors in CdTe thin film. First, the sputtering-deposited CdTe thin film was immersed in AgNO3 solution for ion exchange method to dope Ag ions. The effects of immersion temperature and Ag-concentration were investigated on the optical properties and electrical characteristics of CdTe thin film by using Auger electron spectroscopy (AES) depth-profile, UV-visible spectrophotometer, and a Hall effect measurement system. The best optical and electrical characteristics were sucessfully obtained by Ag doping at high temperature and concentration. The larger and more uniform diffusion of Ag ions made increase of the Ag ion density in CdTe thin film to decrease the series resistance as well as mede the faster diffusion of light by the metal ions to enhance the light absorption. Second, Ag was doped by using helium-neon (He-Ne) laser (632.8 nm) exposure into sputtering-deposited p-type CdTe thin films. The resistivity of the Ag-doped CdTe thin films was reduced from 2.97×104 Ω-cm to the order of 5.16×10-2 Ω-cm. The carrier concentration of CdTe thin films had increased to 1.6×1018cm-3after a 15-minute exposure to the He-Ne laser. The average absorbance value of CdTe thin films was improved from 1.81 to 3.01 by the doping of Ag due to impurity-scattering. These improved properties should contribute to the efficiency of the photovoltaic effect of the photogenerated charged carriers. The methodology in this study is very simple and effective to dope a multilayered thin film solar cell with a relatively short process time, no wet-process, and selective treatment. Laser-induced doping method was chosen to dope Al as a donor into CdTe thin films to perform the selective and controllable doping process for the multilayer structured photovoltaic devices. laser-induced doping of Al used He-Ne laser exposure into the sputtering-deposited p-type CdTe thin films after annealed at 400 oC for 1hour. The AES depth profile showed the better doping uniformity in the longer exposure time of He-Ne laser in doping process. The conductivity of CdTe thin films was changed from p-type to n-typeonce the doping of Al was performed. The optical band gap energy of CdTe thin films decreased from 1.451 eV to 1.418 eV after 10 minutes of the Al doping. The average absorbance value of the CdTe thin films improved from 1.58 to 1.72 by doping with Al due to impurity-scattering via the doped Al atoms in the CdTe thin films. The resistivity of the Al-doped CdTe thin films was reduced significantly from the order of 104 Ω-cm to the order of 10-6 Ω-cm at only 1 minute of exposure. The carrier concentration of the CdTe thin films increased to a maximum value of 1.2×1021 cm-3after a 10-minutes exposure to the He-Ne laser. The laser-induced doping of Al into CdTe thin films is very simple and effective to dope a multilayered thin film solar cell with a relatively short process time, no wet-process, and selective treatment to control the conductivity type, optical band gap energy, absorbance, carrier concentration, and resistivity of the thin films.